This application is based upon and claims benefit of priority of Japanese Patent Application No. 2002-78731 filed on Mar. 20, 2002, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor dynamic sensor such as an acceleration sensor for use in an automotive vehicle.
2. Description of Related Art
An example of a conventional acceleration sensor for use in an automobile is shown in FIG. 4. The acceleration sensor 110 includes a package case 101, a circuit chip 103 and a semiconductor sensor chip 105 supported on the circuit chip 103 via an adhesive film 104. The circuit chip 103 is mounted on the package case 101 with an adhesive 102 in which beads 109 made of resin are dispersed. The beads 109 have a high elasticity modulus of bout 5 giga-Pascal (5 GPa) to maintain a certain space between the circuit chip 103 and a mounting surface of the package case 101.
A beam structure including movable electrodes 156 that detect an amount of acceleration imposed thereon is formed on the semiconductor sensor chip 105. Signals representing the detected acceleration are processed in the circuit chip 103 and fed to an outside circuit. When heat is applied to the package case 101, for example, in a re-flow soldering process, the heat is transferred from the package case 101 to the circuit chip 103 and the semiconductor sensor chip 105. Since thermal expansion coefficients of those components are different from one another, a thermal stress is generated in the semiconductor sensor chip 105. The sensor characteristics change due to the thermal stress.
To suppress transmission of the thermal stress from the circuit chip 103 to the semiconductor sensor chip 105, the adhesive film 104 having a low elasticity modulus (about 1 MPa) is interposed therebetween. The thermal stress transferred to the circuit chip 103 from the package case 101 is not suppressed by the adhesive 102 because it contains the beads 109 having a high elasticity modulus. Although transmission of the thermal stress from the circuit chip 103 to the semiconductor sensor chip 105 is suppressed by the adhesive film 104, it cannot be completely interrupted. Because the semiconductor sensor chip 105 is highly sensitive to the thermal stress, its characteristics are affected by the transferred thermal stress even if its amount is small.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved semiconductor dynamic sensor having stable characteristics, avoiding thermal stress transfer to a semiconductor sensor chip.
A semiconductor dynamic sensor such as an acceleration sensor is composed of a semiconductor sensor chip, a circuit chip for processing outputs from the sensor chip and a package case for containing the sensor and circuit chips therein. The circuit chip is mounted on a mounting surface of the package case, and the sensor chip is supported on the circuit chip. In order to prevent a thermal stress from being transferred from the package case to the sensor chip through the circuit chip, an adhesive film having an elasticity modulus lower than 10 MPa is interposed between the package case and the circuit chip.
Preferably, another adhesive film having a low elasticity modulus is interposed between the sensor chip and the circuit chip. The adhesive film interposed between the package case and the circuit chip may be made of silicone or the like.
In this manner, thermal stress transfer from the package case to the semiconductor sensor chip through the circuit chip is suppressed, and thereby characteristics of the semiconductor sensor chip are kept stable.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.